1. Field of the Invention
The invention relates to a glass ceramic cooking plate which exhibits increased transmission for visible light in a localized zone as compared to adjacent zones, and also relates to a method for modifying absorption properties of glass ceramic material in a localized area or full surface area over at least part of the thickness and/or volume of the starting material by a treatment with electromagnetic radiation, so that transmission of the monolithic starting material is altered in a desired extent. Specifically, the invention relates to glass ceramic cooking plates which can be produced by the method according to the invention.
2. Description of Related Art
For locally altering the transmission of glass ceramic components, four different possible ways have been known so far:
First, by joining two different materials with different transmission, a component can be created, which partially has a different transmission. Any joining process may be used for this purpose, such as brazing, welding and gluing. A drawback hereof is that in this case two different materials of different transmission are needed which have to be produced individually, and setting of a specific and different transmission is a challenge or is even not realizable at all in many cases. Moreover, the two different materials may have different mechanical, physical, and chemical properties. This might be disadvantageous in later use in terms of thermal shock resistance, chemical resistance, and mechanical fracture resistance. In addition, the joining seam has different physical and chemical properties and may have a detrimental effect on the properties of the component. In addition, the joining seam usually is visually disruptive or constitutes a starting edge for a fracture. Moreover, incorporation of closed surfaces into a huge component is often very difficult, since the joining has to be accomplished on all sides and gap sizes are difficult to be met, and it is impossible to apply forces on the joining seam to enhance adherence.
Second, transmission may be altered by a local coating. Such a solution is proposed in WO 2012/001300 A1, for example. Although only one material is required in this case, in contrast to a joining method, a coating material is additionally required, which has to meet specific required transmission characteristics. In order to obtain a locally higher transmission, those areas of the component which are to exhibit lower transmission are coated. A prerequisite is that the entire component must have a basic transmission that is as high as the highest transmission required in the finished product. In practice, this may lead to increased cost and complexity, since possibly the glass composition has to be changed.
Also, the cost and complexity of partial coating must not be underestimated, since masking has to be done in any way. Another drawback of the coating method is that a suitable coating has to be found, which sufficiently adheres on the component and survives any later operating conditions of the component without getting damaged.
Furthermore, the coating creates a new surface on the component, with different chemical and physical properties. In case of an outer coating of an article, scratches or other alterations and damage may occur. In addition, a coating always builds up on the surface, which is often undesirable in terms of haptics, appearance, scratch susceptibility, or friction.
Third, from EP 0233146 B1 a method is known for marking ceramic materials, glazes, glass ceramics and glasses by means of a laser. In this case, inorganic pigment particles in form of a “ceramic color body” are added as a radiation-sensitive additive to the material to be inscribed, which additive takes a different color as a result of the laser radiation. Since such pigment particles may only be added to glasses and glass ceramics during melting, they would also melt and would not have an effect any more. Such a method is only conceivable in ceramics which are sintered from powders. Also, a pulsed and focused laser beam which acts superficially is indispensable, since the ceramics are not transparent. In this process, the optimum wavelength to be selected for irradiation is that which is absorbed best by the radiation-sensitive additive, but the least possible by the inorganic material which is to be marked. Therefore, a prerequisite is a locally different absorption of the starting material, which means that local absorption points have to be included in the ceramics, which lead to a locally different absorption (and hence different color impression) of the starting material to be marked. Also, the depth of penetration of this markings is usually not greater than about 1 mm, since ceramics are usually opaque.